An impregnated dispenser cathode for use in a large projection tube and an extra-large cathode ray tube for an HDTV is manufactured in such a manner that an electron emissive material such as barium calcium aluminate is impregnated into a porous metal matrix of a high melting point. The typical examples of such an impregnated dispenser cathode are disclosed in U.S. Pat. No. 4,165,473 and U.S. Pat. No. 4,400,648. The usual forms of these conventional impregnated dispenser cathodes, as shown in FIG. 1, comprise a porous metal matrix 10 in which electron emissive material is impregnated, a cup 20 for storing it, and a sleeve 30 for supporting and securing the cup 20 at the upper part thereof and also for receiving a heater 40 inside it.
The manufacturing procedure of the conventional impregnated dispenser cathode described above comprises the following steps.
1) A porous metal matrix manufacturing step: a porous metal matrix is obtained by sintering a body consisting of compressed powdery tungsten and molybdenum powder. PA1 2) An impregnating step: while an electron emissive material such as barium calcium aluminate is in contact with the entire surface of the porous metal matrix, the electron emissive material is fused and impregnated into the metal matrix within a hydrogen atmosphere of a vacuum. PA1 3) A residue removing step: the residue which has become stuck to the surface of the porous metal matrix during the impregnating step is removed by shot peening. PA1 4) Parts fixing step: the three parts, i.e., the sleeve, cup, and metal matrix are fixed to one another by a laser welder after the porous metal matrix is put into the cup specially manufactured and inserted in the upper portion of the sleeve.
In the conventional impregnated dispenser cathode manufactured by the above steps, as shown in FIG. 1, the welding is carried out by the beam from a laser welder or resistance welding heat applied to the upper end of the sleeve 30. The three parts are secured to one another simultaneously at one welding point W1. Electron emissive material impregnated into the metal matrix may inadvertently be evaporated in part when the metal matrix melts during this conventional way of welding.
The electron emissive material evaporates because heat energy is applied so intensely that the three parts, sleeve, cup, and metal matrix, are simultaneously fused at the welding point W1. Further, at this time, the sleeve and other parts may be so heavily damaged as to make the end product unusable.
If lower energy is applied to the welding point in order to obviate the above problem, the welding of the three parts is not completely accomplished failing to secure the parts. In particular, if the metal matrix is not completely secured, it is apt to come out of the cup.
In addition, during the residue removing step, the metal matrix becomes excessively abraded when peening particles collide against the entire surface of the metal matrix. As explained above, this full scale shot peening is needed because a residue of electron emissive material is stuck to the entire surface of the porous metal matrix during the impregnating step where the entire surface of the porous metal matrix is in contact with electron emissive material; thereby negatively affecting manufacturing.